docs: def plot_residuals(self, plot=None):
          """ Plot normalized fit residuals.
  
          The sum of the squares of the residuals equals ``self.chi2``.
          Individual residuals should be distributed about one, in
          a Gaussian distribution.
  
          Args:
              plot: :mod:`matplotlib` plotter. If ``None``, uses
                  ``matplotlib.pyplot`.
  
          Returns:
              Plotter ``plot``.
          """
          if plot is None:
              import matplotlib.pyplot as plot
          x = numpy.arange(1, len(self.residuals) + 1)
          y = _gvar.mean(self.residuals)
          yerr = _gvar.sdev(self.residuals)
          plot.errorbar(x=x, y=y, yerr=yerr, fmt='o', color='b')
          plot.ylabel('normalized residuals')
          xr = [x[0], x[-1]]
          plot.plot([x[0], x[-1]], [0, 0], 'r-')
          plot.fill_between(
              x=xr, y1=[-1,-1], y2=[1,1], color='r', alpha=0.075
              )
          return plot